1. Field of the Invention
The invention relates to photolithography for fabricating micro devices on a substrate, and particularly to determining substrate plane tilt and image plane tilt of the exposure apparatus in a photolithography system and, more particularly, to determining such tilts of a scanning-type exposure apparatus.
2. Related Background Art
One of the processes involved in the fabrication of micro devices (such as semiconductor devices, liquid crystal display devices, plasma display devices and the like) is photolithography. Generally, this involves imaging a mask pattern formed on a reticle onto a predetermined shot area on a photo sensitized substrate (a wafer or glass plate with photoresist applied thereon), and subsequently developing and processing the exposed substrate to obtain the surface features represented by the mask pattern. Two exposure methods adopted for such photolithography processes are the projection-type exposure method and the scanning-type exposure method.
In the projection-type exposure method, the image of the entire pattern on the reticle is projected onto the substrate at one time. By stepping the substrate, the same image may be repeatedly projected onto sequential areas of the substrate. The projection exposure apparatus for repeating the stepping and projection exposure is called a step-and-repeat apparatus (or stepper).
In the scanning-type exposure method, the reticle and the substrate are typically scanned and exposed synchronously (in accordance with the image reduction, if any, in place) with respect to an illumination area defined by a slit having a predetermined geometry (e.g., a rectangular, hexagonal, trapezoidal or arc shaped slit). This allows a pattern larger than the slit-like illumination area to be transferred to a shot area onto the substrate. After the first shot area has been completed, the substrate is stepped to position the following shot area to a scanning start position. This system of repeating the stepping and scanning exposure is called a step-and-scan system. The scan-type exposure method is especially useful for imaging large reticle patterns and/or large image fields on the substrate, as the exposure area of the reticle and the image field on the substrate are effectively enlarged by the scanning process.
For both exposure methods as well as others, it is important that the photosensitive surface of the substrate be in focus with respect to the projection optics. This is to ensure sharp definition and resolution of the surface features formed by the lithographic process. Focus errors may arise from misalignment of the best focal plane or image plane of the projection optics and the plane of the substrate surface. Such misalignment may arise from imperfections in the orientation of the projection optics (resulting in image plane tilt with respect to the horizontal plane) and/or the orientation of the substrate on its support (resulting in substrate plane tilt with respect to the horizontal plane). The misalignment between the image plane and the substrate plane would result in optimum focus for only part of the entire image field. In view of the impetus to obtain increasingly larger scale integration of surface features at increasingly higher resolution, it is imperative that the image of the reticle be accurately and reliably focused over the entire area of each image field.
Some prior art exposure devices have been provided with an auto-focus feature that automatically adjusts the focus of the reticle pattern onto the substrate surface. However, this feature only ensures that the image field is at overall optimum focus with respect to the projection optics. This feature does not ensure that the entire image field is in optimum focus. For example, in a scanning-type lithography system, since the exposure through the slit at a given instant is an area that is typically relatively narrow in the direction of scan and long in the orthogonal direction, it would be more difficult to ensure that the entire image field is in focus along the entire longitudinal dimension of the image field. For this and other reasons, auto-focus does not work as well for scanning-type exposure systems compared to projection-type exposure systems. The difficulty in achieving focus over the entire image field is exacerbated by the relatively shallow depth of field of the exposure optics because of its relatively large numerical aperture.
Some prior art exposure apparatuses were provided with means for determining the image plane tilt and means for offsetting the misalignment between the substrate surface and the image plane (e.g., by adjusting the substrate tilt position), to minimize focus errors over the entire image field. For example, it is common practice in the semiconductor industry to calibrate various aspects of the lithography system by exposing test patterns on a test substrate. One of the primary aspects to be determined is the optimum focus at different locations on the substrate. Typically, a series of exposures are made on the test substrate, involving changing focus distance and shifting the pattern laterally between exposures to not overlap exposures. The images formed by these exposures are analyzed later to determine the locations on the test substrate which are in optimum focus. Based on these locations, the projection system can be adjusted in terms of overall focus as well as the tilt of the image plane in one or more degrees of freedom.
The above calibration test may be carried out based on the assumption that there is no substrate plane tilt. Such a calibration test may be effective for determining the location of the optimum image plane and the image plane tilt if there is no substrate plane tilt. However, since there is the possibility that the substrate is also tilted, there would be ambiguity in the calibration results with respect to the uncertainty between substrate tilt and image plane tilt. It is therefore desirable to provide a calibration test that can resolve this ambiguity. It is further desirable to provide a calibration test that is simple to carry out in a photolithography process.
The invention provides a method for determining and calibrating substrate plane tilt and image plane tilt in a photolithography system. This method includes subjecting a test substrate to exposures to form images from which substrate plane tilt can be distinguished from image plane tilt and the two types of tilt can be separately determined. In one aspect of the invention, the presence of substrate plane tilt about a first axis (e.g., Y-axis) is identified by making a series of exposures on a test area with the test substrate shifted, relative to the reticle, along the horizontal reference plane (i.e., zero tilt plane; e.g., X-Y plane) in a direction along a second axis (e.g., X-axis) orthogonal to the first axis. The presence of substrate plane tilt is demonstrated by a shift in the relative locations of the best focus images within the test area. The relative locations of the best focus images within the test area would remain substantially the same if there were no substrate plane tilt about the first axis.
In another aspect of the invention, a multiple exposure series is developed to separately determine image plane tilt and substrate plane tilt about the first axis. At least two exposure series are made in at least two test areas aligned along the second axis direction on the substrate plane. For a first exposure series, the two test areas are subject to the same exposures at the same position along a third axis (e.g., Z-axis) orthogonal to the horizontal reference plane. The image from this exposure series would indicate the presence of substrate plane tilt, based on a comparison of the relative positions of the best focus images within the two test areas. For a second exposure series, the substrate plane is stepped along the third axis direction and one of the test areas is subjected to the same exposure at different positions in the third axis direction. The image from this second exposure series provides information on the change in position of best focus across the substrate plane, corresponding to changes in substrate position along the third axis direction. Such information is used for determining the image plane tilt with respect to the substrate plane.
In the event that substrate plane tilt is present, the image from the second exposure series also provides information relating to the change in position of the substrate plane along the third axis that would render the best focus image in the test area that was subjected to the second exposure series the same as the best focus image in the other test area. The substrate plane tilt about the first axis with respect to the horizontal reference plane is given by the change in position in the third axis direction necessary to obtain the same best focus image in each test area and the distance between the best focus images in the two test areas.
The image plane tilt and substrate plane tilt about the second axis is determined by a similar approach, using two test areas aligned along the first axis. To determine the tilts about the first and second axes at the same time, three test areas may be arranged on the substrate plane in a manner that would separately resolve the tilts about the first and second axes.
The absolute tilt of the image plane about the horizontal reference plane is given by the algebraic sum of the substrate plane tilt angle (relative to the horizontal reference plane) and the image plane tilt angle (relative to the substrate plane). The image plane tilt and substrate plane tilt information may be used to calibrate the photolithography system for processing production substrates.